In its simplest form an ion mobility separator (IMS) comprises a pulsed source of ions and a drift tube containing a buffer or drift gas. An electric field or travelling DC wave is applied along the drift tube so as to urge ions from an ion entrance to an ion exit of the drift tube. As the ions traverse the drift tube they separate according to their mobility through the buffer or drift gas. The velocity (v) of an ion having an ion mobility (K) in a drift tube with an applied electric field (E) is given by:v=KE
Such devices have been constructed using ion guides in which the ions are confined by electrodes. AC voltages that oscillate at RF frequencies are applied to the electrodes so as to create a pseudo-potential force that confines the ions and allows highly efficient ion transmission through the ion guide.
Ions are typically pulsed into the IMS drift tube for analysis at periodic intervals that are spaced apart sufficiently to allow ions from one pulse to pass through the drift tube before ions in the next pulse enter the drift tube. In order maximise the duty cycle of such a device, it is known to receive and trap the ions from an ion source in an ion trapping region upstream of the IMS drift tube in the durations between the times that the ions are pulsed into the drift tube. Fewer ions are lost since ions are accumulated in the upstream ion trap during the time that a previously accumulated packet of ions is traversing the IMS drift tube. The separation time of a first pulse of ions in the IMS device is synchronised with the trapping and release times of the next pulse of ions into the IMS drift tube. This allows an ion transmission efficiency of nearly 100% to be achieved.
However, it is desired to provide an improved method of ion mobility separation and an improved ion mobility spectrometer, which preferably provide improved IMS peak widths and more reproducible drift times through the IMS drift tube.